docs: def pypackable(name, pytype, format):
      """
      Create a "mix-in" class with a python type and a
      Packable with the given struct format
      """
      size, items = _formatinfo(format)
      return type(Packable)(name, (pytype, Packable), {
          '_format_': format,
          '_size_': size,
          '_items_': items,
      })